ZDC controller misjudges required separation and descends a B733 conflicting with a B752.

Attributes
ACN	510039
Time
Date	200105
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : rdu.airport
State Reference	NC
Altitude	msl bound lower : 29000 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc.artcc tracon : phl.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	510039
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation non adherence : published procedure
Independent Detector	atc equipment : conflict alert
Resolutory Action	none taken : detected after the fact
Consequence	faa : investigated
Miss Distance	horizontal : 22800 vertical : 600
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Aircraft #1 at FL330 en route to bwi. Aircraft #2 FL310 en route atl. I was waiting for aircraft #1 to pass aircraft #2, so I could descend aircraft #1 to FL290. Aircraft were on opposite courses. After coordination with another sector on another matter, I went back and gave aircraft #1 descent clearance. Conflict alert was activated after aircraft #1 left FL330 on back side of aircraft #2.

Original NASA ASRS Text

Title: ZDC CTLR MISJUDGES REQUIRED SEPARATION AND DSNDS A B733 CONFLICTING WITH A B752.

Narrative: ACFT #1 AT FL330 ENRTE TO BWI. ACFT #2 FL310 ENRTE ATL. I WAS WAITING FOR ACFT #1 TO PASS ACFT #2, SO I COULD DSND ACFT #1 TO FL290. ACFT WERE ON OPPOSITE COURSES. AFTER COORD WITH ANOTHER SECTOR ON ANOTHER MATTER, I WENT BACK AND GAVE ACFT #1 DSCNT CLRNC. CONFLICT ALERT WAS ACTIVATED AFTER ACFT #1 LEFT FL330 ON BACK SIDE OF ACFT #2.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.